1. Field of the Invention
The present invention relates to packaging for optically interactive electronic devices. More particularly, the present invention relates to package assemblies for solid-state image sensors wherein a transparent cover is adhesively attached over the active surface of an image sensor chip and the image sensor chip is sealed within an encapsulant. The present invention also relates to methods of fabricating the image sensor package assemblies in an efficient manner to eliminate or reduce the potential for deposition of contaminants on the image sensor chip active surface during the fabrication process.
2. State of the Art
Optically interactive electronic devices, for example, charge coupled device (CCD) image sensors or complementary metal-oxide semiconductor (CMOS) image sensors, are typically enclosed within a package for subsequent connection to higher-level packaging such as a larger circuit assembly in the form of a carrier substrate. The package provides electrical interconnection to the larger circuit assembly, provides protection from the surrounding environment and allows light or other forms of radiation to pass through to sensing circuitry located on the image sensor device. In the prior art, package formation has often been accomplished by placing an image sensor device in the form of a semiconductor chip into the cavity of a plastic or ceramic housing, wire bonding electrical connection points on the semiconductor chip to conductive pads associated with the housing and sealing a window or transparent cover over the cavity. The materials and structure involved with this packaging technique require a fabrication process that can be time consuming and requires several precision assembly steps. Further, each assembly step increases the opportunity for contamination or damage to the image sensor device itself, raising defect levels and slowing production time to avoid such damage and contamination. Due to the extremely cost-competitive nature of today's semiconductor industry, even small improvements in product yield and production time are of value, especially when considered in terms of the high volume of components being manufactured.
In response to large-scale production requirements, various attempts have been made to simplify the construction of image sensor packaging. U.S. Pat. No. 6,351,027 to Giboney et al. and U.S. Pat. No. 6,285,064 to Foster, for instance, disclose wafer-level packaging formed by laying a solid sidewall piece or an adhesive matrix over a wafer having an array of sensor devices and covering it with one or more transparent top pieces. The wafer is then singulated to create discrete chip-scale packages. While these packaging techniques reduce material costs and assembly steps, they do not completely protect the semiconductor chips from the environment and may require complicated process steps for forming electrical connections to the sensor devices. U.S. Pat. No. 6,266,197 to Glenn et al. discloses a method for forming image sensor packages wherein an array of image sensors is wire bonded to a carrier substrate, and a molded window array is placed over the array of image sensors. The substrate and attached molded window array are then singulated to form a plurality of individual image sensor packages. The molded window array of Glenn et al., however, suffers from the fact that individual transparent windows must be formed within or later attached to the molded array, requiring additional assembly and alignment steps during fabrication. U.S. Pat. No. 5,811,799 to Wu discloses an image sensor package formed by attaching a plurality of preformed or glue walls to an array of printed wiring frames having image sensors thereon and sealing the walls with transparent material. The printed wiring frames are then diced to form discrete packages. Once again, this arrangement may require the attachment or formation of multiple components during fabrication and may increase the occurrence of process contamination.
As is evident from the foregoing description of the state of the art, a simplified method is needed for forming an image sensor package that is conducive to mass production while still offering suitable and robust mechanical and environmental protection for an image sensor chip.